Improvement of the grip or comfort have become important priorities for a large number of tire ranges. Consequently, it is desirable to produce blends having good mechanical properties and an increased hysteresis in a certain temperature range in order to be able to use them for manufacturing semi-finished products that are incorporated in the composition of tire casings, such as for example underlayers, sidewalls or treads and to obtain tires that have an improved grip or comfort.
It is known that the degree of branching can influence certain properties of the polymers. Branched polymers can be prepared by radical, cationic, Ziegler-Natta, metallocene or anionic polymerization. Branched polymers can have various structures depending on the reaction conditions used.
Thus, it is known from the publications by Knauss et al., Polym. Prep. ACS. Div. Poly. Chem. 38(1), pages 68-69 (1997), Polym. Prep. ACS. Div. Poly, Poly Chem. 41(2) pages 1397-1398 (2000), J. Poly. Sci. Polym. Chem. 38, pages 4289-4298 (2000), J. Poly. Sci. Polym. Chem. 38, pages 3547-3555 (2001), Macromolécules, 35, pages 2055-2062 (2002), how to react a living anionic polymer, in this case polystyryllithium, with a coupling agent to form a macromonomer in situ which in turn reacts with the living chains present. The living chains thus formed react with the newly introduced coupling agent and this continues until the coupling agent is completely consumed. The reaction with a difunctional or tetrafunctional electrophilic agent leads to highly branched “pom-pom” or “pom-star” polymers, the arms of which may themselves be branched or connected to linear-dendritic hybrids when the coupling agent is added slowly.
Document U.S. Pat. No. 4,996,266 discloses the synthesis of branched copolymers, the arms (or grafts) of which are arranged along the main chain, either randomly or at predetermined locations.
These copolymers are obtained by the synthesis of a living elastomer obtained by reaction of a lithium initiator with a polymerizable monomer and a so-called organometallic condensing agent bearing a vinyl functional group; the lithium initiator is used at a sub-stoichiometric molar ratio relative to the condensing agent, then reaction with a multifunctional coupling agent.
These branched copolymers, that can be used as adhesives, have a structure which approaches a random and dendritic structure since there is simultaneously formation of the macromonomer and polymerization of the polymerizable monomer.
Document WO 02/16490 A2 describes a branched polymer having a comb structure with a large number of side arms that confer damping properties. This document also describes the use of this comb-structured branched polymer in tire treads.